Add properties: axes_r, axes_w, & axes_c

[prjemian]

Could add properties for axes_r (real-space axes as defined by libhkl that are used to calculate forward reflection) and axes_w (the axes to be updated by forward calculation). The axes in axes_r that are missing from axes_w are the ones held constant (a question posed recently by @strempfer), a new property called axes_c.

If the engine module has these properties, they would use the canonical names. Then, the calc module could also define the same properties and replace canonical with renamed axes. Makes it easier for the user to understand a given mode and engine.

Originally posted by @prjemian in #306 (comment)